There are a large number of eye-movement measuring techniques in the art and the principal ones are disclosed in the Young & Sheena articles which are incorporated by reference herein. This invention relates to those eyemovement measuring techniques which use an external light source, generally at near infrared wavelength, which is reflected from some portion of the eye to obtain a measurement of eye position or fixation. Generally, such techniques are classified as corneal reflection per se, corneal reflection-pupil center, corneal reflection-double Purkinje image, pupil tracking per se, limbus (i.e., the boundary between the iris and the sclera) tracking, eyelid tracking and combinations thereof. When used throughout this specification, reference to "eye tracker" or "eye tracker means" or "eye tracker mechanism" means any and all conventional mechanisms which utilize any of the aforementioned tracking techniques principally be measuring reflection of light from or over a portion of the eye. This is in distinction to electrooculography and contact lens eye-movement measurement techniques which do not fall within the definition of an eye tracker as used herein.
Eye trackers of the type to which this invention relates, may be further classified as (i) head-mounted, in the sense that the principal measurement instruments are secured by a helmet or head band to the observer's head or (ii) "remote" or floor mounted in the sense that no instruments are applied to the observer's head even though chin rests or other devices might be used to immobilize the observer's head movement of (iii) a combination of "head-mounted" and "remote" or "hybrid" devices which do not exist in a practical, commercial sense, but are present in any theoretical consideration.
A totally "remote" eye tracker system is produced by Applied Science Laboratories, the assignee of the present invention, in its 1996 and 1998 model lines which are further described in our U.S. Pat. No. 4,755,045, U.S. patent application Ser. No. 848,154, and in ASL's Eye Trac Catalog, all incorporated by reference herein. In the 1998 model, the position of a servo controlled tracking mirror is controlled to maintain the eye image within the eye camera field-of-view so that eye line-of-gaze can be determined with the pupil center to corneal reflection technique. In this manner, rapid, unrestrained movements of the head will not result in loss of eye measurement even with as much as one foot of lateral or vertical head motion. Because there are no head-mounted instruments nor any other distracting instrumentation present to the observer, the 1996 and 1998 systems are ideal for eye tracking measurements where the observer is seated, such as in the cockpit of a flight trainer or in a chair watching video commercials, etc. However, there are countless research, industrial and military applications where it is desired to accurately see what a person is looking at instead of projecting a predetermined scene and monitoring the reaction of the observer to the projected scene. Such applications typically use headmounted systems to monitor eye-movements.
As noted, hybrid head-mounted - remote systems exist in the literature. For example, in EPC application No. 0157973, the external light source for directing the near infrared light for eye measurement purposes is mounted in the observation room while the corneal reflection instrument is attached to eyeglass frames affixed to the observer, who is viewing a scene projected on a screen. In U.S. Pat. No. 4,034,401, both the near infrared light source and the eye tracker camera (which is of the limbus tracking type) are reflected off a pilot's helmet to locate the eye position relative to an externally generated weapons pointing display reflected on the windshield of the aircraft. In both applications, the observer is seated or stationary and looking at a scene which is projected in front of him. To partially mount some of the eye tracker mechanism to the head of the observer simply encumbers the observer without presenting any enhancement of the system when compared with the ASL 1998 model used either in an airplane cockpit environment or in a seated environment for viewing artificially projected scenes such as commercials and the like. For such reasons, "hybrid" eye measurement systems are not commercially practical.
This then leaves head-mounted systems to satisfy those applications, i.e., observer movement and/or real life scene viewing, which cannot be addressed by head-free systems. A head-mounted, eye monitoring system as thus defined herein requires a field-of-view or scene camera which records any external scene as actually viewed by the observer and an eye tracker mechanism, both items secured by an appropriate head band or helmet to the head of the observer. Different, head-mounted systems have been developed in the art for different eye measuring techniques, principally limbus tracking and corneal reflection.
One typical limbus tracking arrangement uses eyeglasses with an infrared source of illumination mounted at the bottom of the lens and flanked on either side by photo cells which electrically record the light reflected to generate an eye image. A field-of-view camera is then added to the eyeglasses to obtain a point-of-gaze display. Examples of such head-mounted limbus tracking systems may be found in ASL's Eye Trac Catalog and in several embodiments disclosed in European Patent Application No. 0,125,808, which also discloses use of CCD chips for imaging. As noted by Young & Sheena, the eyeglass limbus tracking arrangement is suitable for some applications, but is limited with respect to vertical eye-movement measurement. Also, the field-of-view camera is mounted on one side of the eyeglass frame while the eye position measurement instruments are located on the other side and this side-by-side mounting arrangement introduces a parallax error, which may or may not present a problem.
To obtain more precise eye measurement over both horizontal and vertical eye-movement, corneal reflex cameras have been used in head-mounted eye monitoring systems which also employ field-of-view cameras to obtain point of gaze information from an observer having freedom of movement. As disclosed in the Young & Sheena articles, early head-mounted corneal reflex eye monitoring systems used a periscope arrangement with the bottom of the scope carrying the infrared light source and scope lenses which reflected the infrared image to the top of the scope. The top of the scope was mounted on top of the observers head and carried the scene lens and an eye tracker camera in combination with a beam splitter prism for superimposing the corneal reflection as a spot of light onto the scene recorded from the field-of-view camera. Because of difficulties encountered in maintaining the infrared light source appropriately centered relative to the cornea, this concept has been modified into a side-by-side arrangement where the field-of-view camera is mounted on one side of the observer's head while the eye tracker mechanism with appropriate optics is mounted on the opposite side. Fiber optics have been used to lighten the helmet weight. One example of such an arrangement is disclosed in U.S. Pat. No. 3,542,457, incorporated by reference herein. As best illustrated in U.S. Pat. No. 3,542,457, a dichroic fixed mirror is used to reflect light from an infrared lamp to the eye spot or eye track camera for subsequent superimposition on the scene viewed by the field-of-view camera, the eye also viewing the scene through the dichroic mirror which is transparent to visible light. As in the earlier periscope version of the helmet, U.S. Pat. No. 3,542,457 uses a complicated optic system to reflect the light to the cornea and back to the eye tracker camera.
It should also be noted that in the literature, specifically for one of the embodiments disclosed in EPA No. 125-808-A, the concept of using an eye tracker camera on the "limbus tracking" eyeglass frame for recording corneal reflection without complicated optics is used. However, that disclosure fixed the infrared lamp to the bridge of the eyeglass frame and would be suspect to the errors and inaccuracies of the earlier corneal reflex head-mounted systems which used a light source simply positioned in front of the eye.
In addition, it is known and disclosed in ASL's Eye Trac Catalog and discussed in some length by Young & Sheena that any number of different sensors, i.e., magnetic head, optic, mechanical, etc., may be applied to the observer's head to measure the orientation of the eye in space to obtain the point-of-gaze (the angle of gaze relative to a reference point in the visual field) relative to ground.
In summary, the limbus tracking eyeglasses are limited in their ability to measure eye-movement and the helmet mounted corneal reflection cameras require optics which somewhat tend to distort the spot image projection and require extensive calibration and readjustment. More importantly, all head-mounted, eye-movement measurement systems heretofore mounted the field-of-view or scene camera short distance from the eye (or eyes) whose movement was being recorded in a manner which introduced a perspective or parallax error. The parallax error could allow the field-of-view camera to see an object which is actually hidden and thus not visible to the observer. This difference in field-of-view is significantly noticeable at short distances and somewhat insignificant at infinity. When an eye tracker is used with the field-of-view camera in a head-mounted system, the system must be calibrated to the scene distance viewed if accurate point-of-gaze data is to be obtained. That is the field-of-view scene recorded must be adjusted for parallax for the distance of the particular viewed scene and the eye tracker than adjusted relative to the adjusted field-of-view scene thus recorded if accurate point-of-gaze information in space which is depended on absolute eye position, is to be obtained. Heretofore, mechanical and/or optical conflicts have either resulted in camera incompatibility with a head-mounted eye tracker or limitations of eye tracker performance to a specifically calibrated distance.